Damaging Agents

Leslie A. Viereck

Damaging Agents

Eastern dwarfmistletoe (Arceuthobium pusillum) is a destructive disease of black spruce in the Lake States and eastern Canada, but it appears less often in the West and is completely absent in northwestern Canada and Alaska (18). In most areas, infection by mistletoe results in reduced vigor, clumped branches (witches' brooms), and deformed trees; but in some stands it may kill many trees. Successful control is possible by incorporating control methods in the silvicultural management (37).

Several rusts of the genus Chrysomyxa infect both the buds and needles of black spruce. The infection usually remains at low levels but occasionally becomes epidemic and causes defoliation, reduced vigor, and even death of seedlings, saplings, and trees. The cone rust (Chrysomyxa pirolata) often results in greatly reduced seed production but does not kill the tree.

Other diseases of black spruce include a needle cast fungus (Lophodermium spp.), which may cause defoliation and death in local areas; a yellow rust witches' broom (Chrysomyxa arctostaphyli); and a snow blight (Lophophacidium hyperboreum), which may cause extensive damage to black spruce growing in nurseries or young regeneration in the field.

White pocket rots of roots and stems, most commonly Inonotus tomentosus, occur in black spruce and may cause significant damage in some upland stands (4,54).

The spruce budworm (Choristoneura fumiferana) is one of the insects most damaging to black spruce, even though black spruce is less susceptible than red spruce (Picea rubens), white spruce, and balsam fir. Budworm defoliation for several years in succession may result in moderate to severe mortality. The budworm and several other insects often cause serious damage to the flowers or cones, resulting in reduced seed crops (50).

The European spruce sawfly (Diprion hercyniae) is an important pest in eastern Canada but has not invaded western portions of the range. The yellowheaded spruce sawfly (Pikonema alaskensis) and greenheaded spruce sawfly (P. dimmockii) occasionally defoliate black spruce but seldom cause serious damage over large areas. Occasionally, a buildup in populations of the spruce beetle (Dendroctonus rufipennis) in white spruce leads to invasion and death of black spruce, usually where the two species are growing together. The spruce bud midge (Rhabdophaga swainei) may affect height growth in black spruce under some conditions (7). Monochamus wood borers have been known to kill considerable numbers of trees in areas adjacent to strip cuts as a result of initial buildup of populations in logging slash (50).

Snowshoe hare may cause extensive damage to seedlings and saplings when populations of hare are high. Red squirrels gather cones in large quantities and give a peculiar clumped appearance to the top of the tree. Squirrels and microtines may consume a large percentage of the seed supply in some areas during poor seed years.

Black spruce tops are often broken at a height of 3 to 6 m (10 to 20 ft) by snow and ice. In Alaska, one storm in 1967-68 broke 28 percent of the stems in a 160-year-old black spruce stand (46). Windthrow and breakage are two of the principal causes of mortality in black spruce stands in the Lake States; they must be considered when planning for harvesting black spruce stands.

Black spruce is easily killed by both ground and crown fires. It generally rates high in fire hazard, although many peatland stands have a low risk except during very dry periods (26).

Black spruce growing in peatlands is especially susceptible to changes in the water table, which sometimes occur naturally as the result of damming of small streams by beavers, but also result from increased or impeded drainage caused by road construction.